Counting the number of stars in the known universe would seem to be a monstrous task akin to counting the number of grains of sand on all the world’s beaches. Of course, no one literally “counts” each and every star, or grain of sand. To enumerate such vast quantities, science must rely on its best estimates. Better instrumentation will often change those previous estimates.
Quite recently, there has been a recalculation of the total number of stars (and their “star factory” galaxies) in the visible universe. The long-standing estimate, made famous via Carl Sagan’s book/PBS tv series Cosmos (“billions and billions”), was that there were approximately one hundred billion galaxies, each containing roughly one hundred billion stars. Some later estimates have suggested one trillion such galaxies.
However, it seems that these numbers are off by several multiples. Previous estimates used spiral galaxies (such as our Milky Way, and the Andromeda galaxy) as the standard gauge for star counting. Yet, about one third of our universe’s galaxies are not spiral types, but rather, egg-shaped types known as elliptical galaxies (note: there are other types too, but most are variations of these two kinds).
Recent calculations of the content of eight elliptical galaxies — specifically, the estimate of older, M-type, or red dwarf, stars within them — conducted by Yale University astronomer Pieter van Dokkum and Harvard astrophysicist Charlie Conroy, have found the previous estimates of red dwarfs to be off by a factor of three–totaling trillions more stars. Extrapolating from these eight elliptical galaxies, the researchers estimate that all such galaxies may be home to 10 to 2o times the number of red dwarfs as found in spiral galaxies, and this in turn triples the total number of stars in the known universe.
If you want a mind-bogglingly huge number to contemplate, try this on: 300 sextillion stars! (a sextillion is a billion times a trillion, or the number 10 followed by 23 zeros).
So, now it’s “sextillions and sextillions”…at least. Oh, and that’s vastly more stars than the number of grains of sand on all the world’s beaches. Oddly, another calculation of the total number of cells in all the world’s cellular lifeforms comes to just about 300 sextillion.
Methinks the spirit of Carl is still smiling, still in amazement at our incredible cosmos.
Red dwarf stars (in our galaxy) are also the home stars to a large percentage of recently discovered exoplanets (such as Gliese 581c, and recently, 581g), which are planets existing outside our solar system. Previous theories held that such red dwarfs did not have enough mass to generate planetary systems. But the growing number of exoplanet discoveries (nearly 400 by now) are demonstrating that red dwarf stars are ideal places to look for more Earth-like planets beyond our solar system.
Also in cosmic news from November: astrophysicists at the European Southern Observatory (ESO), in Chile, using the Very Large Telescope (VLT) detected the most distant galaxy in the universe. It’s called galaxy UDFy-38135539 and resides some 13, 000 million light years away, thus making it also the oldest known galaxy. It is not clear if other such “most distant” galaxies were detected and/or included in the recent, upward-adjusted, total star estimate.
These newest star count calculations were included in the paper A substantial population of low-mass stars in luminous elliptical galaxies published in last week’s edition of the journal Nature.
All images come from apod.nasa.gov
Note to the Reader: an earlier version of this post stated an under-estimation of stars of “3 trillion times”. This error was the result of an abbreviation of two separate facts. It has been corrected. my apologies. — M.R.